Microsoft Visual Basic 2010 Step by Step eBook

242 Part II Programming Fundamentals

The error handler has responded to the disc drive problem by allowing the user a few

tries to fix the problem, and then it has disabled the problematic button. (In other

words, the user can no longer click the button .) This disabling action stops future

run-time errors, although the program might no longer function exactly as it was

originally designed.

9. Click the Close button on the form to stop the program.

Using Nested Try Catch Blocks

You can also use nested Try... Catch code blocks in your error handlers. For example, the

following disc drive error handler uses a second Try... Catch block to retry the file open

operation a single time if the first attempt fails and generates a run-time error:

Try

PictureBox1.Image = _

System.Drawing.Bitmap.FromFile("d:\fileopen.bmp")

Catch

MsgBox("Insert the disc in drive D, then click OK!")

Try

PictureBox1.Image = _

System.Drawing.Bitmap.FromFile("d:\fileopen.bmp")

Catch

MsgBox("File Load feature disabled")

Button1.Enabled = False

End Try

End Try

If the user inserts the disc in the drive as a result of the message prompt, the second Try

block opens the file without error. However, if a file-related run-time error still appears, the

second Catch block displays a message saying that the file load feature is being disabled,

and the button is disabled.

In general, nested Try... Catch error handlers work well so long as you don’t have too many

tests or retries to manage. If you do need to retry a problematic operation many times, use

a variable to track your retries, or develop a function containing an error handler that can

be called repeatedly from your event procedures. (For more information about creating

functions, see Chapter 10 .)

Comparing Error Handlers with Defensive

Programming Techniques

Error handlers aren’t the only mechanism for protecting a program against run-time errors. For

example, the following program code uses the File.Exists method in the System.IO namespace

of the .NET Framework class library to check whether a file exists on CD or DVD before it’s

opened:

If File.Exists("d:\fileopen.bmp") Then

PictureBox1.Image = _